There are many chemical processes where it is necessary to bring into contact a fluid and a solid particulate matter, such as adsorbents and catalysts. Frequently, chemical reactions as well as physical phenomena occur for a predetermined period of time in the contact zone, e.g. a reaction or adsorption zone. In many of these processes, the particles are transported between two or more particle containing vessels. The particles may be transported for a variety of reasons depending on the process. For example, particles may be transported from one contacting vessel or zone into another contacting zone in order to take advantage of different process conditions to improve product yields and/or purity. In another example, particles may be transported from a reaction zone into a regeneration zone in order to rejuvenate the particles, and after rejuvenation, the particles may be transported back to the reaction zone. The particles may be introduced to and withdrawn from the vessels or zones in a continuous or semi-continuous manner sufficient to maintain the desired contacting process continuously.
The vessels between which the catalyst is transported are not necessarily adjacent. The outlet of the source vessel from which the catalyst is transported may be a significant distance horizontally and/or vertically from the inlet of the destination vessel to which the catalyst is transported. Pneumatic conveying through a conduit is a well known and commonly used method of transferring catalyst over vertical and horizontal distances. One characteristic of pneumatic conveying is that because of the pressure difference across the conduit between the source and destination, the destination pressure must be less than the source pressure to account for the pressure drop across the pneumatic conveying system. However, process conditions may require the destination vessel to operate at a higher pressure than this value (source pressure minus pneumatic conveying system pressure drop). Examples include circulating particles between two zones maintained at different pressures; and transferring particles from one vessel to another where both vessels are maintained at the same pressure. Under such conditions, a pneumatic conveying system alone is insufficient to transfer the particles.
A lock hopper is commonly used to transfer particles from a lower pressure zone to a higher pressure zone. The use of lock hoppers in conjunction with pneumatic conveying is also well known in the art to transfer particles between vessels or zones that are maintained at different pressures. First, a lock hopper transfers particles from the upper, low pressure source zone to a middle zone, and then to a lower, high pressure zone. A pneumatic conveying system then transfers the particles from the high pressure zone to the destination zone. Although the destination zone has a pressure less than that of the high pressure zone, the destination zone pressure may be greater than that of the low pressure source. In the art, the term “lock hopper” has been used to designate the combination of the upper, middle, and lower zones, and “lock hopper” has been used to designate only the middle zone.
In one example, the flow of particles from an upper vessel into the middle zone and out of the middle zone into a lower zone is controlled by valves located in the conduits or transfer pipes that connect the zones. The valves may be double block-and-bleed ball valves. Thus, a batch of particles may be transferred to the middle zone through the upper valve or valves when the lower valve or valves are closed. The middle zone may then be isolated by closing the upper valve(s). Various conduits may be connected to the isolated volume to introduce or remove the fluid phase, usually gas, or change the pressure inside the middle zone. For example, a regenerated catalyst may enter the vessel, be purged with nitrogen to remove oxygen, and pressured with hydrogen before being transferred to the reactor which is at a higher pressure than the regenerated catalyst. After catalyst exits the middle zone, the middle zone can be purged with nitrogen to remove the hydrogen before filling again with catalyst.
U.S. Pat. No. 4,576,712 discloses a method and apparatus for maintaining a substantially continuous gas flow through particulate solids in two zones. The solids are moved from a low pressure zone to a high pressure zone by means of a valveless lock hopper system. Maintenance of gas flow while simultaneously transferring particles between zones is accomplished without the use of moving equipment such as valves.
U.S. Pat. No. 4,872,969 discloses a method and apparatus for controlling the transfer of particles between zones of different pressure using particle collection and particle transfer conduits. The solids are moved from a low pressure zone to a high pressure zone by means of a valveless lock hopper system that vents all of the gas from the collection zones through the particle collection conduits. The venting of gas is accomplished by varying the size of the transfer conduits between zones.
As is known in the art, physical characteristics of the particles and basic process information such as the operating pressure in the upper and lower zones and the acceptable range of gas flow rates are initial design information. Processes are designed from this basic information and standard particle and gas engineering principles to routinely provide stable operating units. Surprisingly, it has been found that a particular unit will operate predominantly in a stable manner but experience sporadic upsets. These upsets involving a sudden surge of particles from one zone to another, which may reverse the particle flow, have been unpredictable with respect to which unit will be affected, and which particle transfer cycle will experience an upset in an affected unit. These upsets occur despite conformance to the same design methods. Such upsets interrupt the consistent flow of particles and can physically damage the particles as well as the equipment.
Consequently, there is desire to eliminate these sporadic upsets in order to minimize damage to the equipment and particles and ensure the consistent flow of particles. The consistent flow or transfer of particles involves a series of steps which are repeated in a cyclic manner to transfer the particles in batches. Although it remains unpredictable whether an upset will occur during any particular cycle in an apparatus, we have discovered that the upsets usually occur during the middle zone depressurization step or the middle zone empty step. Our invention provides an improved method and apparatus that eliminates all or many of these sporadic upsets without negatively impacting the vast majority of operating units or cycles that do not experience upsets.